The present invention relates to a paper-manufacturing method and apparatus for conveying a web from a forming wire to a drying section of a paper machine.
Thus, the present invention is concerned with the wet end of a paper machine and in particular with a method and apparatus for detaching the web from a forming wire while transporting the web to a press section and beyond the latter to a drying section, and while carrying out dewatering of the web as it travels toward the drying section.
The detaching of the web formed in the wire section of a paper machine and its transfer to the press section is an extremely important step inasmuch as this particular step has a great influence on the reliable continued operation of a paper machine. With paper machines which operate at relatively low speeds it is possible to provide for open draws where there is an open conduction of an unsupported web, based on a speed differential between the wire and press sections. Such operations are known in connection with paper machines which operate at low speeds.
In paper machines which operate at high speeds or which manufacture thin, low-strength types of paper, it is known to utilize transfer systems which will protect the web, and the present invention relates to systems of this type. Such systems usually operate in such a way that a transfer felt of the press section is guided so as to contact the web at the location where it is to be detached from the wire. By way of the expedient of pressing, with a suitable rotating roll, the transfer felt against the web, the web is caused to adhere to the transfer felt which then transports the web away from the forming wire to the first nip of the press section.
In general there are two main types of closed transfer or carry-over systems. The simplest is the so-called "lick-up transfer", based on the ability of a wet "lick-up" felt to adhere the paper web to its surface. The other type of system is a "vacuum pick-up" system developed from the first system. By utilizing suction at the transfer location it is possible to adhere the web to the transfer felt with greater reliability. The vacuum pick-up system affords greater possibilities, as compared with the first system, for example with respect to selection of felt quality. Particularly in those cases where the transfer felt also operates as a press felt several requirements are imposed on the transfer felt. Thus the web must adhere reliably thereto particularly at the location where the web is detached from the wire, but on the other hand the felt must operate efficiently at the dewatering nip.
Conventional vacuum pick-up systems employ a suction roll and have been widely used. Such systems are encumbered, however, by certain drawbacks referred to below.
Thus, the suction roll utilized for reliably detaching the web from the wire and adhereing the web to the transfer felt has a perforated shell which may cause marking of the web, so as to detract from the appearance of the paper as well as possibly affecting its surface characteristics undesirably. Moreover, such suction rolls are extremely expensive. They require their own driving motor with their own control system, and they create an undesirable noise. Such suction rolls also have the drawback of consuming a large amount of air, because the suction system draws in through the perforations of the suction roll not only air which passes through the web and felt but also air arriving at the suction zone and entrained in the holes of the suction roll shell at every revolution thereof. Furthermore, various difficulties are created by the sealing water of the suction box which is situated in the interior of the suction roll.
In a conventional Fourdrinier machine, detachment of the web from the wire takes place at a location situated on a run of the wire between the couch roll and the traction roll. At this location the web travels together with the wire in a downward direction at an inclination which is on the order of 45.degree. with respect to a horizontal plane. The detaching location is determined by the normal structure of the wire section and press section and by their location with respect to each other. Subsequent to the detaching location the pick-up felt and the web attached thereto lap the pick-up roll through an angle on the order of 70.degree.-90.degree., and the pick-up felt with the web adhering thereto travel on to the press section.
Under certain circumstances (high speeds, inappropriate felt) the change in direction caused by the above lapping of the pick-up roll creates a tendency for the web to become separated from the pick-up felt as a result of the effect of centrifugal force. In order to prevent such separation of the web from the pick-up felt, the latter must be provided with a suction zone which is relatively extensive inasmuch as it must extend substantially beyond the detaching zone proper. In this way it is possible to make certain that the web adheres to the felt, but this requirement also creates an undesirably large additional load on the suction system of the pick-up roll. As a result the suction roll requires an undesirably great suction capacity, far beyond what is required simply for bringing about detachment of the web from the wire and attachment of the web to the felt.
Because of the above factors it has become known also in certain cases to utilize a stationary transfer suction box instead of the suction roll, and by the use of such a stationary transfer suction box it is possible to avoid some of the above drawbacks.
With respect to the state of the art pertaining to the present invention reference may be made to U.S. Pat. Nos. 3,441,476, 3,528,881, and 3,537,955.
As is well known, suction rolls are made with a rotating, perforated shell cylinder the inner surface of which is slidably engaged in a fluid-tight manner by a stationary suction box which extends axially along the interior of the shell from one end to the other thereof and which has a breadth on the order of, for example, 100-150 mm. The suction box is connected to the suction system in such a way that an air flow is achieved through holes which pass through the shell of the suction roll, the suction being created at that area of the shell which at any given instant extends across the suction box which has a sealed engagement with the inner surface of the shell. Such suction rolls are of course expensive. The drilling thereof, in particular, is difficult to carry out and involves high costs. The perforations made by drilling through the shell detract from the strength thereof, so that special metal alloys must be used, and in addition a considerable shell thickness is essential, all of which necessitates high material costs.
The air which is entrained in the holes of the suction roll shell and thereby carried into the suction zone and into the suction system has proved to be of an unexpectedly great quantity in modern, fast-running paper machines. The extent of this "hole air" increases progressively with increasing paper machine speed. The magnitude of the amount of "hole air" is even further increased by the fact that as the machine speed increases the suction rolls must be made of greater strength, and this is accomplished by increasing the thickness of the shell so that the quantity of hole air becomes proportional to the shell thickness.
A particularly serious drawback with respect to such suction rolls is that as they rotate they generate sharp disturbing noise which creates a serious health detriment to the workers in the vicinity of the machine. The generation of this noise results from the fact that the holes in the suction roll act as whistles. As these holes which are under vacuum arrive at the region just beyond the suction zone they are abruptly filled with air, giving rise to a powerful whistling sound which has a fundamental frequency equal to the acoustical resonance frequency of the hole. The set of whistling pipes constituted by the numerous holes of the suction roll often cause a noise surpassing the pain threshold of the human ear. While there have been previous attempts to suppress this noise by way of various arrangements, such as by using a more favorable drilling pattern, the fact is that in practice there has been no significant reduction in this undesirable noise.
In connection with press suction rolls, in particular, it is frequently necessary to provide for compensation for the deflection of the roll, but up to the present time this has not been possible because the hollow interior of the roll is already occupied by the suction box and therefore a deflection-compensating structure, which in itself is known, cannot be accommodated in the interior of such a suction roll.
One of the important operating characteristics which is highly desired in paper machines of the above type is the capability of achieving an effective dewatering action. It is a well-known physical fact that the viscosity of water diminishes considerably with increasing temperature. This factor has been utilized in attempts achieve a more efficient dewatering of the paper web in paper machines. Examples of previously known designs of this type may be seen in U.S. Pat. Nos. 2,907,690, 3,097,995, 3,560,333, and 3,655,507.
However, structures for enhancing the dewatering action by raising the temperature of the felt, web, and/or roll have not gained any extensive use, particularly with respect to fast-operating paper machines. This is partly because it is not possible by way of any known means to supply a great enough thermal energy flow to the points of supply. This latter factor results because the temperaure of treating gas is limited (for example with a view to preventing damage to the felt), the time available for heat transfer is extremely short owing to the high felt and web velocities, and the heat transfer area also is extremely restricted, as a result of space requirements.